1. Field of the Invention
The present invention relates to electrical circuits for supplying power to electrical glow discharge devices. The invention relates especially to circuits for supplying very fast, high power pulses to electric discharge electrodes used to excite gas lasers.
2. Description of the Prior Art
A common application for high power pulsed electric glow discharges is in the excitation or pumping of gas lasers. Generally, the glow discharge is created by applying a voltage across a glow discharge device consisting of a pair of electrodes disposed within an envelope containing the gaseous laser gain medium.
The design of the electrical excitation circuit which supplies current to the discharge electrodes contributes substantially to the overall efficiency of the laser, that is, to the ratio of the laser's optical output power to the electric power consumed by the excitation circuit.
One component of the overall laser efficiency, the efficiency with which the laser gain medium is excited by the electric discharge, is enhanced by minimizing the rise time of the current pulse applied to the discharge electrodes. Another component of the overall laser efficiency, the efficiency of the excitation circuit itself, is enhanced by minimizing power dissipation in each circuit component.
The design of an efficient excitation circuit is complicated by the fact that the voltage required to initiate an electric glow discharge (called the "firing voltage") is much greater than the voltage required to sustain the discharge (called the "sustaining voltage" or "steady-state voltage"). Therefore, the excitation circuit must apply to the discharge electrodes a voltage greater than the predetermined firing voltage for the brief period required to initiate the discharge, and then need only apply a voltage at the lower value needed to sustain the desired level of discharge current for the remaining period in which the discharge is to occur.
Conventional excitation circuits generally consist of a main power supply for supplying the aforesaid voltages and an "output switch" for connecting the power supply to the discharge electrodes. The output switch is needed to disconnect the power supply voltage from the discharge device until the moment the glow discharge is to begin.
The output switch is difficult to design because it must satisfy several requirements: it must conduct the relatively high current of the sustained glow discharge; it must be able to turn on very fast and have a very low inductance to achieve a fast rise time for the discharge current pulse; and it must have a very low "on" resistance to minimize power losses.
The type of switch generally accepted as best satisfying the electrical requirements of an output switch is the multichannel railgap, a sparkgap device typically consisting of a long brass bar positioned parallel to and only a few millimeters from the edge of a long stainless steel blade. However, a serious shortcoming of the railgap is that current flow through the sparkgap progressively erodes the electrodes and deposits electrode material on adjacent dielectric surfaces, so that the railgap has a relatively short lifetime until replacement or repair is required.
Thyratrons and saturable inductors may operate as switches having longer lifetimes than railgaps, but their performance as output switches is inferior because of their slow turn-on speed and high power losses, respectively.